Arbitrary bit-width saturing addition and subtraction operations. Created for regher's Advanced Embedded Systems class.

sat_ops.c

#include "sat_ops.h"
#include <stdio.h>

/* Note: This code assumes that sizeof(myuint) == sizeof(myint). This assumption
 * is made because the comment at the top of 'sat_ops.h' says that each pair
 * will be in effect at the same time. I am sacrificing robustness to acheive
 * a smaller footprint. */

#define  UMIN ((myuint) 0)
#define  UMAX ((myuint) (~UMIN))
#define UHIGH ((myuint) (~(UMAX >> 1)))

#define MIN ((mysint) UHIGH)
#define MAX ((mysint) (UMAX >> 1))

myuint sat_unsigned_sub(myuint a, myuint b) {
    return b > a ? 0U : a - b;
}

myuint sat_unsigned_add(myuint a, myuint b) {
    myuint sum = a + b;
    return sum < a || sum < b ? UMAX : sum;
}

/* splitting this into a seperate function reduces my byte count by 16 bytes
 * in GCC. */
static void signs(myuint *a, myuint *b, myuint *s, 
                  myuint *out_a, myuint *out_b, myuint *out_s) {
    *out_a = (*a & UHIGH);
    *out_b = (*b & UHIGH);
    *out_s = (*s & UHIGH);
}

mysint sat_signed_add(mysint a, mysint b) {
    myuint usum = (myuint) a + (myuint) b;
    myuint a_sign, b_sign, s_sign;
    signs((myuint *) &a, (myuint *) &b, &usum, &a_sign, &b_sign, &s_sign);
    if (!a_sign && !b_sign && s_sign) { return MAX; }
    else if (a_sign && b_sign && !s_sign) { return MIN; }
    return (mysint) usum;
}

mysint sat_signed_sub(mysint a, mysint b) {
    myuint usub = (myuint) a - (myuint) b;
    myuint a_sign, b_sign, s_sign;
    signs((myuint *) &a, (myuint *) &b, &usub, &a_sign, &b_sign, &s_sign);
    if (!a_sign && b_sign && s_sign) { return MAX; }
    else if (a_sign && !b_sign && !s_sign) { return MIN; }
    return (mysint) usub;
}